In fluid metering, the temperature of the flowing medium is an important element or parameter in determining accurately the volume of flow. Normally the conduit includes an orifice through which the fluid medium flows and a suitable fluid meter measures the fluid pressure on opposed sides of the orifice. Flow velocity, line pressure and temperature are necessary in order to measure accurately the flow of the fluid medium through the conduit. If the temperature of the fluid medium is not correct, the calculated flow volume by the meter which includes a computer processing unit (CPU) will be inaccurate.
In measuring the gas flow through a gas pipeline, for example, in which the gas flow volume is utilized to effect payment of gas being purchased, an inaccuracy in the temperature of the flowing gas may result in substantial monetary sums being paid or not being paid for delivered gas resulting from an error in the temperature of the flowing gas. Thus, it is highly important that an accurate temperature measurement be obtained for the flowing gas.
A thermowell assembly which comprises a temperature sensing probe (TSP) received within an outer metallic sheath or tube is commonly used for injecting within the pipeline. A thermowell is used so that a temperature sensing probe (TSP) may be easily removed from the thermowell and replaced. Pipelines normally are buried within the ground but a length portion of the pipeline commonly extends above the ground for mounting of a meter to calculate the flow volume of the fluid transported in the pipeline. While the ground maintains a generally uniform temperature, the pipeline section above the ground is exposed to ambient conditions which may vary greatly. Variations in temperature of the exposed pipeline sections results in a temperature conduction from the metallic pipeline to the metallic thermowell assembly which may result in an inaccurate temperature measurement of the flowing medium.
U.S. Pat. No. 4,510,343 dated Apr. 9, 1985 shows a thermowell apparatus having a thermocouple mounted within an outer closed metallic tube or sheath and extending within a pipe. The outer closed tube or sheath is positioned closely adjacent a metallic pipe and an outer metallic housing mounted on the pipe. Thus, temperature from the pipe exposed to ambient conditions is conducted to the outer tube of the thermowell which may result in an error in the temperature of the fluid flow under certain conditions. U.S. Pat. No. 5,048,323 dated Sep. 17, 1991 is directed to a sensor for line pressure and line temperature including a sealed chamber containing a fixed mass of a reference gas. A vibrating quartz density sensor produces an output signal.
It is desirable that a thermowell assembly be provided for a metallic conduit which is substantially temperature insulated or isolated from the metallic conduit so that any temperature inaccuracies of the sensed flowing medium are minimized or eliminated.